Electra pilosa and Membranipora membranacea are the common mats found on seaweed fronds. Superficially they are very similar but close inspection will reveal one (E.pilosa) comprises quite oval cells, the other (M.membranacea) very rectangular cells.
They both form extensive and easily spotted small carpets, patches and coatings on the surfaces of Seaweeds.
I have chosen to put them both in the same Observation so as to show the main differences